1. Field of the Invention
The present invention relates to an exhaust gas purification device for an internal combustion engine. More specifically, the invention relates to an exhaust gas purification device equipped with an NOx occluding and reducing catalyst for removing NOx components from the exhaust gas of an engine.
2. Description of the Related Art
There has been known an exhaust gas purification device using an NOx occluding and reducing catalyst comprising at least one element selected from alkali metals such as pottasium K, sodium Na, lithium Li and cesium Cs, alkaline earth elements such as barium Ba and calcium Ca, and rare earth elements such as lanthanum La and yttrium Y, as well as a noble metal such as platinum Pt.
The NOx occluding and reducing catalyst works to absorb and release NOx, i.e., absorbs NOx in the exhaust gas in the form of nitrates when the air-fuel ratio of the exhaust gas that flow in is lean, releases the absorbed NOx when the oxygen concentration decreases in the exhaust gas that flow in, and purifies the released NOx by reduction with reducing components in the exhaust gas.
The action for absorbing and releasing NOx by the NOx occluding and reducing catalyst will be described later. However, when sulfur oxides (SOx) exist in the exhaust gas, the NOx occluding and reducing catalyst absorbs SOx in the exhaust gas based on the same mechanism as that of NOx absorption.
However, SOx absorbed by the NOx occluding and reducing catalyst forms stable sulfates which are generally very little decomposed or released and, hence, tend to build up in the NOx occluding and reducing catalyst. When SOx is held in an increased amount by the NOx occluding and reducing catalyst, the NOx occluding and reducing catalyst exhibits a decreased NOx-absorbing capacity by an amount by which SOx is held. When SOx is held in increased amounts by the NOx occluding and reducing catalyst, therefore, it becomes no longer possible to remove NOx from the exhaust gas to a sufficient degree, i.e., a so-called SOx contamination of the Nox occluding and reducing catalyst occurs.
It is also known that SOx absorbed by the NOx occluding and reducing catalyst can be released and reduced relying on the same mechanism for releasing and reducing NOx. However, since sulfates held in the NOx occluding and reducing catalyst are relatively stable, SOx absorbed by the NOx occluding and reducing catalyst cannot be released at a temperature (e.g., about 250.degree. C.) at which the operation is usually conducted to release and purify NOx by reduction (hereinafter referred to as "a regenerating operation" for the NOx occluding and reducing catalyst). In order to remove the SOx contamination, therefore, the SOx contamination-recovery operation must be regularly conducted by heating the NOx occluding and reducing catalyst at a temperature (e.g., 600.degree. C. or higher) which is higher than that when the NOx occluding and reducing catalyst is usually being generated, and by shifting the air-fuel ratio of the exhaust gas that is flowing in toward the rich side.
An exhaust gas purification device which executes the SOx contamination-removing operation for the NOx occluding and reducing catalyst, has been taught in, for example, Japanese Unexamined Patent Publication (Kokai) No. 6-88518. In order to recover the SOx-contaminated NOx occluding and reducing catalyst according to the device of the above publication, the exhaust gas is put into a stoichiometric state or a rich state to lower the oxygen concentration when the temperature of the exhaust gas becomes high.
Japanese Unexamined Patent Publication (Kokai) No. 8-61052 discloses another exhaust gas purification device for recovering the NOx occluding and reducing catalyst from the SOx contamination by regenerating the NOx occluding and reducing catalyst at a high temperature. In order to recover the NOX occluding and reducing catalyst arranged in the exhaust passage of an internal combustion engine from the SOx contamination according to the exhaust gas purification device of this publication, some of the cylinders of the engine are operated at a rich air-fuel ratio and the rest of the cylinders are operated at a lean air-fuel ratio to remove the SOx contamination when the amount of SOx held by the NOx occluding and reducing catalyst becomes greater than a predetermined value and the engine is operated in a particular load region (intermediate to high load region). By operating some cylinders of the engine at a rich air-fuel ratio and the rest of the cylinders at a lean air-fuel ratio, exhaust gas from the rich air-fuel ratio cylinders containing unburned fuel and exhaust gas from the lean air-fuel ratio cylinders containing large amounts of oxygen flow into the catalyst. Therefore, the unburned fuel burns on the catalyst, whereby the temperature of the catalyst rises and SOx held by the NOx occluding and reducing catalyst is released.
According to the devices of the above-mentioned Japanese Unexamined Patent Publications (Kokai) Nos. 6-88518 and 8-610652, however, it is not allowed to frequently conduct the SOx contamination-recovery operation, and a state in which the purification efficiency of the NOx occluding and reducing catalyst is lowered due to the SOx contamination often continues for extended periods of time. According to the devices of the above-mentioned publications, the recovery operation from the SOx contamination is not executed unless the exhaust gas temperature of the engine becomes high or unless the engine is operated under a particular condition even if the amount of SOx held by the NOx occluding and reducing catalyst exceeds a predetermined value.
By using the devices of the above-mentioned publications, therefore, the operation often continues for extended periods of time in a state where SOx are held in increased amounts by the NOx occluding and reducing catalyst depending upon the operating conditions of the engine, and NOx which has not been purified are released to the open air due to a drop in the purification efficiency of the NOx occluding and reducing catalyst.
In conducting the above-mentioned recovery operation from the SOx contamination of the NOx occluding and reducing catalyst, SOx is still formed by sulfur contained in the fuel when the engine is in operation, and a difference occurs in the degree of removing the SOx contamination from the NOx occluding and reducing catalyst depending upon the SOx concentration in the exhaust gas during this period. When the exhaust gas is put into a predetermined stoichiometric state or a rich state, depending on the prior art, without taking the above-mentioned fact into consideration, then, the SOx contamination is not removed to a sufficient degree, or the exhaust gas becomes excessively rich to deteriorate the fuel efficiency and the exhaust gas emission.